1. Field of the Invention
The present invention relates to a light emitting device and, more specifically, to a light emitting device formed of a nitride semiconductor. The light emitting device in accordance with the present invention may simply refer to a semiconductor device or a semiconductor chip mainly formed of a nitride semiconductor substrate and a semiconductor layer stacked thereon, or a device having the semiconductor chip mounted on a mounting component and resin-sealed. The term may also refer to both types of devices. The semiconductor chip may be simply referred to as a chip. Further, of the chip, the substrate and an epitaxial layer formed thereon may simply be referred to as a substrate.
2. Description of the Background Art
A white light emitting diode (LED) is at present popularly used for illumination for small electronic equipment such as a portable information terminal, and it will possibly be used for illuminating a large space or large area. For large space or large area illumination, the LED must have higher optical output. For this purpose, it is necessary to cause a large current to flow through the LED and to solve the problem of increased temperature inherent to heat generation.
FIG. 70 shows a currently proposed structure of a GaN-based LED (Japanese Patent Laying-Open No. 2003-8083). In the GaN-based LED, an n-type GaN layer 102 is formed on a sapphire substrate 101, and between the n-type GaN layer 102 and a p-type GaN layer 104, a quantum well structure 103 is formed. Light emission occurs in quantum well structure 103. On p-type GaN layer 104, a p-electrode 105 is formed to be in ohmic contact, and on n-type GaN layer 102, an n-electrode is formed to be in ohmic contact.
These p- and n-electrodes 105 and 106 are connected to a mounting component 109. The mounting component (sub-mount component) is formed from an Si substrate, and has a circuit for protection against external surge voltage formed thereon. Specifically, in view of the fact that main cause of circuit malfunctions of III-group nitride semiconductor containing Ga, Al, In or the like is a surge voltage such as transient current or electrostatic discharge, a power shunt circuit for protecting the light emitting device is formed by a zener diode or the like, so as to prevent a large forward or backward voltage from being applied to the light emitting device. Protection against the surge voltage will be described in detail later.
The GaN-based LED is characterized in that (a1) p-type GaN layer 104 is mounted face-down so that light is emitted from a back surface of sapphire substrate 101, and (a2) n-electrode layer 106 is formed on n-type GaN layer 102. The structure of GaN-based LED is very complicated, as can be seen from FIG. 70. The reason why (a2) the n-electrode layer 106 is formed on n-type GaN layer 102 is that the n-electrode cannot be provided on the sapphire substrate, as sapphire substrate 101 is an insulator.
Not only in the light emitting device employing a sapphire substrate described above but also in GaAs-based, GaP-based and GaN-based compound semiconductors used for light emitting device, proposals have been often made for providing a protection circuit against transient voltage and electrostatic discharge, within the light emitting device (Japanese Patent Laying-Open Nos. 2000-286457, 11-54801 and 11-220176). Particularly in a GaN-based compound semiconductor, breakdown voltage in the backward direction is as low as about 50 V, and breakdown voltage in the forward direction is no more than about 150 V. Therefore, provision of a power shunt circuit for protection is of much importance. Specifically, the chip of the above-described GaN or other base is formed on a sub-mount Si substrate, and a protection circuit including a zener diode or the like is formed on the Si substrate. Many proposals on protection circuits as described above indicate that the main cause of circuit malfunctions of III-group nitride semiconductor containing Ga, Al, In or the like is a surge voltage such as transient current or electrostatic discharge.
Other than the light emitting device having such a protection circuit as described above, an example has been known in which a GaN-based light emitting device is formed on an SiC substrate that is a conductor. Specifically, an LED having a stacked structure of (n-electrode on back surface of SiC substrate/SiC substrate/n-type GaN layer/quantum well stacked structure (light generating layer)/p-type GaN layer/p-electrode) in which light is emitted from the p-type GaN layer has also been widely used.
Further, an example has also been known in which lattice-like trenches are formed on a substrate to provide a plurality of convex projections, for highly efficient use of light emitted from the LED (Japanese Patent Laying-Open No. 2003-23176).
In the GaN-based LED using a sapphire substrate shown in FIG. 70 described above, the structure becomes complicated, inherently increasing the manufacturing cost. To enjoy increased demand for illumination of wide space, LEDs must be inexpensive. Therefore, such a complicated structure is undesirable. Further, arrangement of p-electrode 105 and n-electrode 106 on the surface of the face-down mounting imposes limits on the area of electrodes, particularly the area of p-electrode. In order to attain high output by causing a large current flow, the area of p-electrode should desirably be as large as possible. Such increase in area is limited in the structure shown in FIG. 70, and hence, the optical output is also limited. Further, from the viewpoint of releasing heat generated by the current, arrangement of two electrodes on one surface is undesirable.
Further, a current flowing through n-type GaN layer 102 in a direction parallel to the substrate meets much resistance, resulting in heat generation and increased driving voltage and hence increased power consumption. Particularly, when the n-type GaN layer is made thin to make shorter the step of film formation, production yield of exposure of the n-type GaN film significantly decreases, in addition to the above-described problems of heat generation and increased power consumption.
Light emitting devices including the above-described device using a sapphire substrate generally have limited area of heat radiation, and have large thermal resistance (increase in temperature caused by unit energy input per unit area), and therefore, it is impossible to inject a large current per one light emitting device. When the sapphire substrate is used, the p-electrode area is limited as described above, and therefore, thermal design usually has only a very narrow margin.
Further, in the GaN-based LED using the sapphire substrate, area for heat radiation is limited, and therefore, it becomes unavoidable to adopt a structure in which p- and n-electrodes are formed in a shape of a comb to increase the area of contact, so as to reduce electrical resistance as much as possible and to reduce heat generation. Processing of such a comb-shaped electrode is difficult and leads to an increased manufacturing cost.
As described above, design of thermal conditions is of fundamental importance in a light emitting device. To attain higher output, limits imposed by the above-described thermal conditions must be overcome, and to alleviate such conditions, complicated electrode shapes must be employed.
There is also the following problem. When the GaN-based light emitting device formed on a sapphire substrate is mounted face-down so that the back surface of the sapphire substrate is used as a light emitting surface, light beams of a prescribed incident angle or larger are totally reflected at an interface between the sapphire substrate and the GaN layer that generated and propagated the light beams, and do not go out, as refractive index of sapphire is about 1.8 and that of GaN is about 2.4. Specifically, light beams of which incident angle is in the range of θ≧sin−1 (1.8/2.4)≈42° are confined in the GaN layer and do not go out. This leads to lower light emission efficiency at the main surface of sapphire substrate. Though light emission efficiency is important, there is a still more problem. The totally reflected light beams propagate through the GaN layer and emitted from the side portion of GaN layer. The amount of totally reflected light is considerably large and the GaN layer is thin, so that the optical energy of the light beams emitted from the side portion has a quite high energy density. Sealing resin positioned at the side portion and irradiated with the light beams is damaged, making shorter the life of the light emitting device.
In a GaN-based LED having the structure of (n-electrode on back surface of SiC substrate/SiC substrate/n-type GaN layer/quantum well stacked structure (light generating layer)/p-type GaN layer/p-electrode) in which light is emitted from the side of p-type layer, it is impossible to efficiently emit light of large output to the outside, as the p-electrode has high optical absorption index. When the coverage ratio is decreased or aperture ratio is increased of the p-electrode in order to increase the amount of light to be emitted, it becomes impossible to cause the current flow thoroughly through the GaN layer, as the p-type GaN layer has high electrical resistance. Therefore, light emission cannot fully be activated in the quantum well structure, resulting in lower optical output. Further, electrical resistance increases, causing the problems of heat generation and electrical capacitance. When the p-type GaN layer is made thick in order to cause uniform current to flow entirely through the p-type GaN layer, the p-type GaN layer absorbs much light, limiting the output.